1. Field of the Invention
This invention relates to sensors for determining the ion concentration in a sample. More specifically, this invention relates to sensors with integrated data storage devices that store data for calibrating the sensor.
2. Description of the Related Technology
The medical diagnostics industry is rapidly shifting toward disposable and precalibrated in vitro diagnostic devices. These devices are inexpensive, generally utilize simple instrumentation, and the tests employing these devices may be performed at the point of care by personnel who are less trained than those who conduct the same tests in a clinical environment using complex equipment.
While the simplicity of the testing is welcome, these devices must compete with the results provided by complex equipment in the clinical laboratories in terms of accuracy and reproducibility. These characteristics generally result from a two point calibration of the sensors utilized in the clinical laboratories. Such precise calibration has not been possible or practical for typical diagnostic devices utilizing precalibrated disposable sensors, which typically employ one-point calibration.
Electrochemical sensors function to measure the presence of an ion in a solution. Examples of ions are: calcium, chloride, hydrogen, lithium, magnesium, potassium and sodium. The actual quantitative measurement of the ion concentration is based on the fact that solutions of different ionic strength, if separated by a membrane, create an electrical potential across the membrane. Ion-selective membranes finction by competitive displacement, wherein an ion of interest in a test solution displaces an ion from a ligand embedded within the membrane. The difference in ion concentration between the two solutions is quantitatively translated into a particular electrical potential that may be measured by an electrode, typically in units of millivolts (mV).
The measured potential is thus used to determine the ion concentration. In many sensors this determination is based on a theoretical ideal relationship between concentration of an ion and the electrical potential created by such a concentration. This is shown as line T (theoretical) in FIG. 1. Devices that base their measurement on the theoretical ideal electrical potential are thus useful only to the extent that the actual measurement is within an acceptable error range of the ideal. The difference between the actual measured potential and the theoretical ideal is a measure of the efficiency of the electrode. A large deviation between the actual and the ideal (inefficiency) renders the sensor unreliable or, in the extreme, useless.
Several factors may contribute to sensor inefficiency. For example, many membranes have a predictable rate of decay when in contact with an aqueous (water-based) ion solution or gel. In most devices, this decay may cause an unacceptable inefficiency within about two weeks. Thus, sensors of that type have a shelf life of less than two weeks between manufacture and use.
Another factor that affects sensor inefficiency is the imperfection of the membrane material, even before any degradation caused by aging. That is, because of the physical limitations of any given membrane, perfect efficiency never exists. For example, in a particular use, a 5% error may be deemed to be the largest acceptable error. A particular production batch of membrane may be tested and found to be 3% away from the ideal, before any degradation occurs in the membrane material.
If the inefficiency is constant across the useful range of ion concentration, the 3% difference of this example may be factored into a compensation formula, which would shift the intercept of the ideal line T, without affecting its slope, to yield the actual electrical potential per ion concentration line A (actual) as shown in FIG. 1. (Note that in the Figures, the graphs are drawn for general illustrative purposes, and are not drawn to any particular scale.) However, the inefficiency may not be constant across a concentration range, but may instead increase or decrease with increasing concentration, i.e., the relationship between electrical potential and ion concentration may be nonlinear. In these cases, there is no simple way to adjust for the inefficiency without further calibrating each membrane batch and adjusting the sensor's conversion ratios accordingly, if possible.
However, even in cases where the quality of the membrane material may be determined and adjusted for, deterioration of the sensor still occurs over time, and such deterioration must be accounted for in addition to the initial properties regarding the imperfections of the sensor. Thus, the disadvantages of existing sensors are evident: a very short shelf life and inaccurate, insensitive measurements.
Accuracy and sensitivity of a sensor are both affected by the deviation between actual and ideal correspondence of measured electrical potential to ion concentration. Clearly, if a sensor is inefficient to a given degree, this has a direct effect on accuracy of its readings. Likewise, the mere fact of having to allow for such an inefficiency introduces an error rate, and measurements that differ by less than the built in error rate are thus not discernibly different. In contrast, in a sensor which would be capable of self-monitoring and calibration, there would be no need to factor in error rates due to degradation or membrane inefficiency, since those values themselves, if determinable, could be used to calibrate the sensor just prior to its use.